Yan, J.J., Chou, M.Y., Kaneko, T., and Hwang, P.P. (2007) Gene expression of Na+/H+ exchanger in zebrafish H+-ATPase-rich cells during acclimation to low-Na+ and acidic environments. American journal of physiology. Cell physiology. 293(6):C184-C1823.
In mammalian nephrons, most of the Na(+) and HCO3(-) is reabsorbed by proximal tubular cells in which the Na(+)/H(+) exchanger 3 (NHE3) is the major player. The roles of NHEs in Na(+) uptake/acid-base regulation in freshwater (FW) fish gills are still being debated. In the present study, functional genomic approaches were used to clone and sequence the full-length cDNAs of the nhe family from zebrafish (Danio rerio). A phylogenetic tree analysis of the deduced amino acid sequences showed that zNHE1~8 are homologous to their mammalian counterparts. By RT-PCR analysis and double/triple in situ hybridization/immunocytochemistry, only zNHE3b was expressed in zebrafish gills, and was colocalized with V-H(+)-ATPase but not with Na(+)-K(+)-ATPase, indicating that HR (H(+)-ATPase-rich) cells specifically express NHE3b. A subsequent quantitative RT-PCR analysis demonstrated that acclimation to low-Na(+) FW caused up-regulation and down-regulation of the expressions of znhe3b and zatp6v0c (H(+)-ATPase C-subunit), respectively, in gill HR cells, while acclimation to acidic FW showed reversed effects on the expressions of these two genes. In conclusion, both NHE3b and H(+)-ATPase are probably involved in Na(+) uptake/acid-base regulation in zebrafish gills, like mammalian kidneys, but the partitioning of these two transporters may be differentially regulated depending on the environmental situation in which fish are acclimatized.